5‐Benzyl‐3‐methylimidazolidin‐4‐one‐Derived Reactive Intermediates of Organocatalysis – A Comforting Resemblance of X‐Ray, NMR, and DFT Solid‐Phase, Liquid‐Phase, and Gas‐Phase Structures

Abstract magnified image The X‐ray crystal structures of three (E)‐1‐cinnamoylidene iminium PF6 salts of 5‐benzyl‐3‐methylimidazolidin‐4‐ones (2,2‐dimethyl‐, cis‐2‐(tert‐butyl)‐, and cis‐2‐styryl‐substituted; 2–4, resp.) are reported (Figs. 3–5). In the 2,2‐dimethyl and in the cis‐2‐styryl derivative, 2 and 4, respectively, a CH bond of the cis‐substituent in 2‐position points to the center of the benzene ring of the benzyl group above the five‐membered ring (Fig. 6, a and b). NMR Measurements (Fig. 8) provide evidence that the same structure is present in solution, and that a fourth derivative of this type, 5 (Scheme), has (Z)‐ instead of (E)‐configuration around the CN bond. In the cis‐2‐(tert‐butyl) derivative 3, the benzyl group is located over the iminium π‐system (Figs. 4 and 6, c). Overlays with DFT‐calculated crotonylidene analogs, A and B, show that the theoretical and experimental structures are almost superimposable (Fig. 9 and Table). The structures are discussed in view of their role as reactive intermediates in organocatalysis and in view of the help synthetic organic chemists may experience from theory.